We have demonstrated that the human leukocyte is very well suited for the study of cholesterol biosynthesis and that the leukocytes of heterozygotes for familial hypercholesterolemia responded with an abnormally high and fast induction of HMG-CoA reductase, the controlling enzyme of cholesterol biosynthesis, when the cells were placed in a medium containing lipid-free serum. The main concern of future studies will be the role of lipoprotein-cholesterol and of endogenously synthesized cholesterol in regulating the synthesis (or activity) of HMG-CoA reductase in human cells. BIBLIOGRAPHIC REFERENCES: Abnormal efflux of cholesterol from heterozygous familial hypercholersterolemic leukocytes. A.M. Fogelman, J. Seager, J. Edmond and G. Popjak, Circulation, 52: Suppl. II, 11-81, 1975, A direct relationship between the amount of sterol lost from rat hepatocytes and the increase in activity of HMG-CoA reductase. P.A. Edwards, A.M. Fogelman and G. Popjak, Biochem. Biophys. Res. Commun. 68, 64, 1976.